Additionally it isn't as obvious as one might think to rule out wrongdoing - what if she was drugged and then thrown into the canal,etc. These investigations take some time, often weeks, and are especially difficult the longer the person is deceased and even more so for bodies that were exposed to water for a long time.

It simply wouldn't be a sensible idea to now declare the case a suicide only to revert the statement later on when something highly unlikely turns up.

Basically the current wording is a hint that the police has some evidence that it was suicidal, but did not rule out all other causes yet.